In a "switched diversity" satellite telecommunications network, a call, or channel, set up with a mobile terminal 6 is conveyed selectively over one of at least two propagation paths 4A and 5A. Propagation path selection is based on minimizing satellite transmission power for the call in question. Thus, the call set up with the mobile terminal 6 is switched to that one of the two propagation paths 4A or 5A, respectively associated with the two satellites 4 and 5, which enables satellite transmission power to be minimized for a given transmission quality. As seen from the terminal, the two propagation paths define between them a propagation time difference equal to the time difference between the respective instants at which data is received after being transmitted synchronously over the two propagation paths 4A and 5A. As a result, the switching procedure for switching between propagation paths must be associated with a synchronization procedure for synchronizing the terminal.
In the prior art, the European Telecommunications Standards Institute (ETSI) Recommendations for the Global System for Mobile communications (GSM) define a handover procedure for handing over between cells as follows. On a channel referred to as the "slow associated control channel" or "SACCH", a terminal sends the reception levels of channels referred to as "broadcast common channels" or "BCCHs" transmitted respectively by the current base station with which the call channel is set up, and by the adjacent base stations. As a function of the various power levels, the infrastructure makes the decision to switch the call from the current base station to a following base station. The current base station then transmits a handover request message to the terminal. The terminal responds by transmitting an access request message to the following base station immediately after receiving a message in a BCCH from said following base station. The following base station responds to the access request message by measuring a propagation time between itself and the mobile terminal, and by transmitting information to the terminal relating to a timing advance that the terminal should apply to its clock relative to what it believes to be the clock of the following station, so that the call data transmitted by the terminal is received by the following base station such that it is time synchronized in a time slot of a frame received by said following base station, taking into account the distance between the following base station and the terminal.
In a satellite network, that prior art solution is not very satisfactory. In such a network, the narrow margin of power allocated to each call in order to guarantee at least a minimum call quality gives rise to a rapid deterioration of the call in the presence of an obstacle, for example. Such rapid deterioration of the channels also applies to the control channels. This might then make it impossible for the terminal to receive a message requesting it to switch a channel from a first propagation path to a second propagation path, and as a result the call might be lost.